The present invention relates generally to oil well production testing and, more particularly, to oil well production testing that separates the oil emulsion, free water and gas phases of the well fluid by gravity segregation for subsequent measurement of each phase separately.
Oil well production testers which operate by gravity segregation are commonly used in heavy oil production operations. Typically, the apparatus comprises a horizontally elongate tank or vessel into which the production fluid is introduced at one end to fill the test vessel and undergo gravity segregation into gas, oil emulsion and water phases which typically occupy the top, middle and bottom thirds of the vessel respectively. At its other end, the test vessel is internally fitted with a weir box whose upper edge is spaced about one-third vessel diameter below the upper surface of the test vessel to establish the liquid/gas interface level of the contained fluids. As the three phases of the produced fluid segregate by gravity, the liquid/gas interface rises to the level of the top of the weir box wall, and the lighter oil emulsion spills over the upper edge of the wall and is so segregated from the free water. Thus, the apparatus commonly operates in a three phase condition consisting of a gas blanket in the top third of the vessel, an oil emulsion layer in the middle third of the vessel and a heavier free water layer in the bottom of the vessel.
Outside of the weir box, the test vessel contains an adjacent oil/water interface probe located at a level approximately one-third vessel diameter up from the floor of the vessel. The oil/water interface probe is operatively coupled to a valve in a discharge line for withdrawing free water from the test vessel, the discharge line being metered for the measurement of the water passing therethrough. The weir box contains a float switch at an elevation above that of the oil/water interface probe, so that as oil emulsion in the weir box rises to a level to actuate the float switch, a valve is actuated to open a metered discharge line for the emulsion. The top of the test vessel includes a chamber into which the gas phase is communicated and is fitted with mechanisms for alternatively introducing make-up gas to or removing gas from the gas blanket in a manner to maintain a substantially constant pressure of the gas within the vessel.
In order for this apparatus to operate satisfactorily, it must maintain the gas blanket at a substantially constant pressure above the upper edge of the weir box. As gas is highly compressible, the pressurized gas pad provides stored energy to expel fluid during the oil and water dump cycles. However, many oil wells produce fluid which is undersaturated in gas. Accordingly, testing of these wells not only requires an initial gas precharge but, also, a constant supply of expensive make-up gas since the undersaturated fluids constantly absorb gas. The gas blanket is a potential safety hazard since, as it is both combustible and under pressure, it presents a constant danger of explosion.
A further disadvantage of this apparatus arises from the size of the weir box, which is necessarily relatively small to accommodate adequate total test vessel volume for gravity segregation of the oil emulsion. Thus, an oil emulsion dump from the weir box during a test sequence is, in turn, small, typically about 4 gallons. It follows that, for a significant portion of its operating time, the oil meter is either accelerating to or decelerating from operating speed, which greatly detracts from meter efficiency. Similarly, a water dump is of short and inefficient duration since it is a function of the oil/water interface falling only a fraction of an inch past the interface probe.
Since the duration of the oil and water dumps is short, the piping and dump valves associated therewith must be of a large size. Accordingly, the oil and water meters tend to run in excess of their rated speed and generate inaccurate readings. The use of flow restrictors in the outlet lines to remedy meter overrunning is impractical since much debris flows through these test vessels and a properly sized orifice would be repeatedly plugged up. Additionally, oil viscosity from the various wells served by a single oil well tester is too variable to allow a single flow restrictor size. Accordingly, not only are the meter readings inaccurate but meter repair costs are high due to meter overrunning.